Register File Backup Queue

ABSTRACT

A register file backup system for use with a computer which processes instructions to generate results which thereby change the visual state of the computer. The computer has a register file with a plurality of addressable locations for storing data. The backup system is adapted to return the visual state of the computer to a previous state if an instruction generates an exception. The backup system utilizes less overhead so as to provide easier register file backup than a comparable software or hardware device. The backup system comprises first means for sequentially storing in program order, address information corresponding to destination locations in the register file where instruction results are to be stored. The first means has first and second outputs for transferring the address information stored therein: the first output being coupled to the register file for transferring a first portion of the address information to the register file, and the second output is used for transferring a second portion of address information for backup storage of the register file contents. The backup system also has a second means coupled to (1) the second output of the first means, for receiving and storing the second portion of the address information, and (2) the register file, for receiving and backup storing further information corresponding to the contents of one or more destination locations in the register file before that destination location is changed according to second portion of the address information. A third means is used for transferring the further information from the second means back to the register file locations according to the second portion of the address information stored in the second means after an instruction generates an exception.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/999,005, filed Nov. 30, 2004, which is a continuation of applicationSer. No. 10/231,152, filed Aug. 30, 2002, now U.S. Pat. No. 6,839,832,which is a continuation of application Ser. No. 10/095,071, filed Mar.12, 2002, now U.S. Pat. No. 6,697,936, which is a continuation ofapplication Ser. No. 09/229,172, filed Jan. 13, 1999, now U.S. Pat. No.6,374,347, which is a continuation of application Ser. No. 08/771,911,filed Dec. 23, 1996, now U.S. Pat. No. 5,881,216, which is acontinuation of application Ser. No. 08/403,527, filed Mar. 13, 1995,now U.S. Pat. No. 5,588,113, which is a continuation of application Ser.No. 07/846,237, filed Mar. 5, 1992, now U.S. Pat. No. 5,398,330.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the management of the program-visiblemachine state of computers, and more particularly, to a computerregister file system and method adapted to handle exceptions whichprematurely overwrite register file contents.

2. Related Art

A more detailed description of some of the basic concepts discussed inthis application is found in a number of references, including MikeJohnson, Superscalar Microprocessor Design (Prentice-Hall, Inc.,Englewood Cliffs, N.J., 1991); John L. Hennessy et al., ComputerArchitecture—A Quantitative Approach (Morgan Kaufmann Publishers, Inc.,San Mateo, Calif., 1990). Johnson's text, particularly Chapter 5,provides an excellent discussion of register file exception handling.

Supporting exception handling and in particular precise interrupts,presents a complicated set of problems for the computer architect. Forexample, the result of a particular instruction cannot be written to acentral processor unit's (CPU) register file, or any other part of theprogram-visible machine state, until after it can be determined that theinstruction will not signal any exceptions. Otherwise, the instructionwill have an effect on the visible state of the machine after theexception is signaled. (The terms CPU, computer and processor will beused interchangeably throughout this document).

Historically, this problem has been circumvented by increasing thenumber of processor pipeline stages (pipeline depth) so that the writedoes not occur until after the latest exception is determined. However,this reduces the allowable degree of instruction interlocking and/orincreases the amount of bypass circuitry required, either of whichtypically degrades overall performance.

The concept of a “history buffer” is described by J. E. Smith et al.(“Implementation of Precise Interrupts in Pipelined Processors”,Proceedings of the 12th Annual International Symposium on ComputerArchitecture (June 1985), pp. 36-44), as a means for implementingprecise interrupts in a pipeline scalar processor with out-of-ordercompletion. In this approach, the register file contains theprogram-visible state of the machine, and the history buffer storesitems of the in-order state which have been superseded by items oflookahead state (i.e., it contains old values that have been replaced bynew values; hence the name history buffer).

The history buffer is managed as a circular buffer. Each entry in thehistory buffer is assigned an entry number. There are n entries in thehistory buffer, where n corresponds to the length of the longestfunctional unit pipeline. A head and a tail tag are used to identify thehead of the buffer, and the entry in the buffer reserved for theinstruction, respectively. Entries between the head and tail areconsidered valid.

At issue time, each history buffer entry is loaded with: (1) the valueof the register file prior to the issuing of the instruction, andcontrol information including: (2) a destination register of the result,(3) the program counter, and (4) either an exception bit or a validitybit, depending on whether an exception is generated at the time ofissue.

A Result Shift Register is used in conjunction with the history bufferto manage various machine control signals, including a reorder tag whichis required to properly restore the state of the machine due toout-of-order completion. The result shift register includes entries forthe functional unit that will be supplying the result and thedestination register of the result. The result shift register isoperated as a first-in first-out (FIFO) stack.

Results on a result bus from the processor's functional unit(s) arewritten directly into the register file when an instruction completes.Exception reports come back as an instruction completes and are writteninto the history buffer. The exception reports are guided to the properhistory buffer entry through the use of tags found in the result shiftregister. When the history buffer contains an element at the head thatis known to have finished without exceptions, the history buffer entryis no longer needed and that buffer location can be re-used (the headpointer is incremented). The history buffer can be shorter than themaximum number of pipeline stages. If all history buffer entries areused (the buffer is too small), issue must be blocked until an entrybecomes available. Hence, history buffers are made long enough so thatthis seldom happens.

When an exception condition arrives at the head of the history buffer,the buffer is held, instruction issue is immediately halted, and thereis a wait until pipeline activity completes. The active buffer entriesare then emptied from tail to head, and the history values are loadedback into their original registers. The program counter value found inthe head of the history is the precise program counter.

The extra hardware required by this method is in the form of a largebuffer to contain the history information. Also the register file musthave three read ports since the destination value as well as the sourceoperands must be read at issue time.

In view of the forgoing, it is clear that a simplified backup system istherefore required to handle exceptions.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a register file backup queue systemand method for use with a computer which processes instructions togenerate results which thereby change the visual state of the computer.The computer has a register file with a plurality of addressablelocations for storing data. The backup system of the present inventionis adapted to return the visual state of the computer to a previousstate if an instruction generates an exception. The backup systemutilizes less overhead so as to provide easier register file backup thana comparable software or hardware device.

The present invention sequentially stores in program order in a resulttag queue, address information corresponding to destination locations inthe register file where instruction results are to be stored.

From the result tag queue, a first portion of the address information istransferred to the register file and a second portion of addressinformation is transferred to a backup queue for backup storage of theregister file contents.

The backup queue also receives and stores further informationcorresponding to the contents of one or more destination locations inthe register file before that destination location is changed accordingto said second portion of said address information.

The present invention transfers said further information from saidbackup queue back to the register file locations according to saidsecond portion of said address information stored in said backup queueif an instruction exception is generated.

Before an instruction is retired, the value of any program-visible statethat an instruction may modify (including, but not limited to, the priorvalue of the register file destination register) is read such that allinstructions up to and including the previous instruction have takeneffect prior to the read. The resulting data are placed in the backupqueue that, in effect, “remembers” the program-visible state of theprocessor exactly prior to any given “uncommitted instruction”, and thuscan be used to nullify the effect of any instruction that causes anexception. (An “uncommitted instruction” is defined by Hennessy et al.as an instruction that may cause an exception at some future time.)

The present invention thus provides a mechanism by which interrupts canbe supported for exceptions that are signaled after the result iswritten and without out-of-order completion. Design complexity isminimally increased, in that the pipeline depth of the processor doesnot need to be increased to handle the late-exception case. Thisapproach is easier to “tack on” to an existing design (e.g., in the casethat an enhancement makes the late-exception case possible where it wasnot possible before) than increasing the pipeline depth. In someconfigurations, overall performance is not significantly impacted,except in the case that an exception occurs.

The foregoing and other features and advantages of the present inventionwill be apparent from the following more particular description of thepreferred embodiments of the invention, as illustrated in theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The invention will be better understood if reference is made to theaccompanying drawings in which:

FIG. 1 shows a representative block diagram of a register file backupqueue system of the present invention.

FIG. 2 is a representative flow chart showing the basic steps taken bythe register file backup queue system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The operation of the invention is illustrated with reference to arepresentative block diagram shown in FIG. 1.

In this example, the computer register file system environment of thepresent invention as shown in FIG. 1 comprises a register file 102having a plurality of addressable locations for storing instructionoperands and results, functional units and bypass logic 104, a resulttag queue 106 for storing information corresponding to the address ofthe register file location that is designated by the instruction tostore results (also called result tag), a backup queue 108, a backupdata latch 110, a backup tag latch 112, a data multiplexer 114 and anaddress/tag multiplexer 116.

In the above example it is easiest to think of the block 102 as aregister file of a central processor unit (also called the processor,CPU or computer), but the present invention applies to any part of (orthe entirety of) the program-visible machine state without loss ofgenerality.

Register file 102 includes a P_adr input to receive informationcorresponding to the address(es) of the register(s) storing theoperand(s) of the instruction (also called operand tags). The operandsare stored in register file 102 and are passed to functional units 104via a bus 134 for execution of the instruction in a conventional manner.

In addition to reading the operands of the instruction, the systemdetermines the destination register to receive the results of theinstruction. The system then reads the contents of that destinationregister prior to execution of the instruction and stores the contentsand a tag (e.g., the address of the register file entry, for example)corresponding to the destination register into backup queue 108. In theevent of an exception (e.g., an arithmetic exception such as an addoverflow), the state of register file 102 prior to the exception can berestored by reading the contents of backup queue 108 and storing thatdata back into register file 102 at the location specified by thecorresponding tag.

To coordinate backup of data in register file 102 a result tag queue 106is required. Result tag queue 106 stores tags corresponding to thedestination location in register file 102 for storing instructionresults. Result tags are generated by decoding the instructions in aconventional manner. The tags stored in result tag queue 106 are storedin a first-in-first-out manner in execution order. Result tag queue 106delays the tag until the previous instructions have completed in casethey write to the same destination location in the register file.

The depth of backup queue 108 depends on the number of pipeline stages(i.e., cycles) between the cycle the register file is written and thecycle that an exception is detected.

One embodiment of the present invention comprises a three stage pipelinewith fetch, decode and execute/store operations. In this embodiment, anexception can only be signaled during the same cycle that register file102 is written. Therefore, backup queue 108 only needs to be deep enoughto store the contents of one destination register and its tag.

Conventionally, if several instruction results are written to registerfile 102 and an exception occurs after a multi-cycle pipeline delay, theresult is that many states of the machine would be wiped out by the latesignaled exception. In the present invention, however, by deepening thebackup queue 108 the lost states can be recovered in order to be able torestore those lost states. Before the exception can be handled, allprevious results (those in the register file before the instructionexecuted) must be written back into register file 102 up to theinstruction preceding the instruction that generated the exception.

As appreciated by those skilled in the art, signals that indicatewhether a data exception (for example) has occurred originate in thedata path and are detected by control logic (not shown). Control logicthen determines what to do and generates a control signal 118 (shown inFIG. 1) which selects one of the sets of inputs of MUXs 114 and 116.

Under normal operation (i.e., no exception), control signal 118 controlsMUX 114 to select “instruction result data” 122 from functional units104 to pass the results to register file 102's write port W.

If an exception is detected, control signal 118 controls MUX 114 toselect “unwrite data” 124 from backup queue 108 to return the state ofthe register file to the state that it was in just prior to theinstruction that caused the exception. The unwrite data 124 is that dataon the top of the backup queue 108.

Similarly, under normal operation (i.e., no exception), control signal118 controls MUX 116 to select a result tag from the top of result tagqueue 106 to a write address input W_adr of register file 102. If anexception is detected, control signal 118 controls MUX 116 to select an“unwrite tag” 125 from backup queue 108 to instruct register file 102where to store the unwrite data.

The backup of data will now be discussed. As shown in FIG. 1, registerfile 102 also includes an input port “Q_adr” for receiving informationconcerning instruction operands from result tag queue 106. The Q_adrinput port receives a “backup tag” 126 from result tag queue 106corresponding to the data that is to be written into the backup queue108. In this example, a bus 128 is shown connected to the entry ofresult tag queue 106 that is to be written to the register file 102 inthe next cycle. At the same time a result tag is sent to the Q_adr inputit is sent via bus 128 to backup queue 108 and is temporarily stored ina tag latch 112. This permits the present invention to store the resultsto be overwritten by the next instruction in backup queue 108. Registerfile 102 outputs the “backup data” 130 corresponding to the backup tag126 via a data output port “Q”. Backup data 130 is sent to a data latch110 so that its corresponding backup tag 126, which arrived earlier attag latch 112, can be latched into backup queue 108 at the same time.

A control line 132 is provided to switch the direction of stacking ofbackup queue 108. In normal operation, backup data and backup tags arepushed into backup queue 108. As noted above, the depth of backup queue108 depends on the length of the pipeline. In this example the backupqueue 108 has a depth of one. Depending on its depth, old information issaid to be “pushed out of the bottom” of the stack when the last entryis written over by newer information. If an exception is detected,control signal 132 reverses the direction of backup queue 108 andinformation is popped off in a last-in, first-out fashion, and is storedback into register file 102.

The process of unwriting can be controlled either by hardware or by theoperating system in software. However, if the operating system does it,then care must be taken to ensure that the instructions that control theprocess do not write into backup queue 108 themselves; that is, theremust be a mechanism for disabling the queue before the exception handleris invoked.

In a preferred embodiment of the present invention, an additional readport (i.e., in addition to the P port) is not required because the readstage and write state are separated by exactly one cycle, and one of thefollowing conditions is satisfied:

-   -   1. There are instructions that cannot generate late exceptions        that require a read port in addition to those required by        instructions that can generate late exceptions, and the depth of        backup queue 108 is exactly 1 (i.e., the latest exception        signals exactly one cycle too late to abort the corresponding        write).    -   2. Every instruction that requires the maximum number of read        ports always writes to the same register number as one of its        arguments.

If one of the previous conditions is satisfied, but the read stage andthe write stage are separated by more than one cycle, then the presentinvention is still applicable without increasing the number ofregister-file read ports. In this case, it is necessary to read intobackup queue 108 at the read stage instead of the stage before write,and to deepen backup queue 108. This, however, is not optimum becausethe additional interlock (from the result of a previous instruction tothe backup read) may degrade performance.

As an example of the present invention, consider a machine with 5pipeline stages A, B, C, D, and E, as shown below in Table 1. Arepresentative flow chart of this example is shown in FIG. 2. Table 1and FIG. 2 will be referred to jointly in the following discussion. Thewrite occurs in stage D. An exception can occur in state E. Exceptionsthat signal after stage C of an instruction cannot prevent thatinstruction from writing. (Reads can occur in any of stages A, B or Cwithout relevance to this example.) Backup queue 108 of the presentinvention must be two storage locations deep because exceptions cansignal two cycles too late to prevent the corresponding write.

In Table 1, the cycle numbers 1-9 are shown along the top, andinstructions i through i+4 and backup queue storage locations Q₁ and Q₂are shown along the left hand side.

TABLE 1 Example Pipeline with Backup Queue Cycle Number 1 2 3 4 5 6 7 89 I* A B C D E i + 1* A B C D E i + 2* A B C D E*** i + 3* A B C D i +4* A B C Q₁** i − 3 i − 2 i − 1 i i + 1 i + 2 i + 3 i + 3 i + 2 Q₂** i −4 i − 3 i − 2 i − 1 i i + 1 i + 2 i + 2 *Instruction Number **BackupQueue ***Exception Signaled

The backup and exception detection of instruction i+2 is shown in FIG.2. The operation starts at a step 202. Instruction i+2 is backed up intobackup queue Q₁ at stage D in cycle 6, as shown in a step 204. Theresult of instruction D is sent to the register file, as shown at a step206. Suppose an exception is caused by instruction i+2 and signals incycle 7 (see the yes branch “Y” at a conditional step 208). At thispoint, the instructions in stages A, B (not shown in cycle 7) and C areaborted (see a step 210), but the results of all instructions throughinstruction i+3 have already been written. However, from the program'sviewpoint, instructions i+2 and i+3 should not have affected the statebecause of the exception. The state overwritten by these twoinstructions is stored in the backup queue (see a step 212). The data inQ₁ is written back into the register file at the destination address ofinstruction i+3 in cycle 8 (see a step 214). Then the backup queueshifts in the reverse direction and the data in Q₁ is written back atthe destination of instruction i+2 in cycle 9 (see a step 216). Theentire visible state now has an appropriate value for the firstinstruction of the exception handler to be fetched and executed (see astep 218).

Note that if instruction i+2 did not signal an exception in stage E andinstruction i+3 signaled an exception in stage D, then the process wouldbe much the same, except that instruction i+2 would not be nullified.(See loop 221 comprising steps 222 and 224.) If no exception isdetected, the pipeline is merely advanced, as shown in a step 226, andexecution continues.

In a further embodiment of the present invention, it is possible to usethis invention to manage part of the visible state while deepening thepipeline for the remainder of the state. The most likely application isto use a backup queue for the register file(s) while delaying the retirefor status bits, since it may be possible to implement bypass logic forthe status bits without degrading performance.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. Thus the breadth and scope of the presentinvention should not be limited by any of the above-described exemplaryembodiments, but should be defined only in accordance with the followingclaims and their equivalents.

1. A register file backup system for use with a computer which processesinstructions to generate results which thereby change the visual stateof the computer, the computer having a register file with a plurality ofaddressable locations for storing data, the backup system adapted toreturn the visual state of the computer to a previous state if aninstruction generates an exception, the backup system utilizing lessoverhead so as to provide easier register file backup than a comparablesoftware or hardware device, the backup system comprising: first meansfor sequentially storing in program order, address informationcorresponding to destination locations in the register file whereinstruction results are to be stored, said first means having first andsecond outputs for transferring said address information stored therein,(a) said first output being coupled to the register file fortransferring a first portion of said address information to the registerfile, and (b) said second output for transferring a second portion ofaddress information for backup storage of the register file contents;second means, coupled to, (i) said second output of said first means,for receiving and storing said second portion of said addressinformation, and (ii) the register file, for receiving and backupstoring further information corresponding to the contents of one or moredestination locations in the register file before that destinationlocation is changed according to said second portion of said addressinformation; and third means for transferring said further informationfrom said second means back to the register file locations according tosaid second portion of said address information stored in said secondmeans after an instruction generates an exception.